Mondeo Zetec

Front lights left and right both make a buzzing noise. When you take the cover off to access the bulbs you can hold a block which makes this noise. The noise appears to start as soon as you turn the side lights on ,also on full beam.
Any ideas how to clear this noise + what is the cause. The car is a Mk 2 X plate mondeo 2lt 16v Zetec. Thanks in advance.